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<div class="titlepage"><div><div><h3 class="title">
<a name="math_toolkit.pol_ref.pol_ref_ref"></a><a class="link" href="pol_ref_ref.html" title="Policy Class Reference">Policy Class Reference</a>
</h3></div></div></div>
<p>
        There's very little to say here, the <code class="computeroutput"><span class="identifier">policy</span></code>
        class is just a rag-bag compile-time container for a collection of policies:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">policies</span><span class="special">/</span><span class="identifier">policy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></pre>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">policies</span>

<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">A1</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A2</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A3</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A4</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A5</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A6</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A7</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A8</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A9</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A10</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A11</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A12</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A13</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">policy</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">domain_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">pole_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">overflow_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">underflow_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">denorm_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">rounding_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">evaluation_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">indeterminate_result_error_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">precision_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">promote_float_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">promote_double_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">discrete_quantile_type</span><span class="special">;</span>
   <span class="keyword">typedef</span> <span class="emphasis"><em>computed-from-template-arguments</em></span> <span class="identifier">assert_undefined_type</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">template</span> <span class="special">&lt;...</span><span class="identifier">argument</span> <span class="identifier">list</span><span class="special">...&gt;</span>
<span class="keyword">typename</span> <span class="identifier">normalise</span><span class="special">&lt;</span><span class="identifier">policy</span><span class="special">&lt;&gt;,</span> <span class="identifier">A1</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">make_policy</span><span class="special">(...</span><span class="identifier">argument</span> <span class="identifier">list</span><span class="special">..);</span>

<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A1</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A2</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A3</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A4</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A5</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A6</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A7</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A8</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A9</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A10</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A11</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A12</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A13</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">normalise</span>
<span class="special">{</span>
   <span class="keyword">typedef</span> computed-from-template-arguments <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
        The member typedefs of class <code class="computeroutput"><span class="identifier">policy</span></code>
        are intended for internal use but are documented briefly here for the sake
        of completeness.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">domain_error_type</span>
</pre>
<p>
        Specifies how domain errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">domain_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">domain_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">pole_error_type</span>
</pre>
<p>
        Specifies how pole-errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">pole_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">pole_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">overflow_error_type</span>
</pre>
<p>
        Specifies how overflow errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">overflow_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">overflow_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">underflow_error_type</span>
</pre>
<p>
        Specifies how underflow errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">underflow_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">underflow_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">denorm_error_type</span>
</pre>
<p>
        Specifies how denorm errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">denorm_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">denorm_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">rounding_error_type</span>
</pre>
<p>
        Specifies how rounding errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">rounding_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">rounding_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">evaluation_error_type</span>
</pre>
<p>
        Specifies how evaluation errors are handled, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">evaluation_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">evaluation_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">indeterminate_error_type</span>
</pre>
<p>
        Specifies how indeterminate result errors are handled, will be an instance
        of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">indeterminate_result_error</span><span class="special">&lt;&gt;</span></code>
        with the template argument to <code class="computeroutput"><span class="identifier">indeterminate_result_error</span></code>
        one of the <code class="computeroutput"><span class="identifier">error_policy_type</span></code>
        enumerated values.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">precision_type</span>
</pre>
<p>
        Specifies the internal precision to use in binary digits (uses zero to represent
        whatever the default precision is). Will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">digits2</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span></code> which
        in turn inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">N</span><span class="special">&gt;</span></code>.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">promote_float_type</span>
</pre>
<p>
        Specifies whether or not to promote <code class="computeroutput"><span class="keyword">float</span></code>
        arguments to <code class="computeroutput"><span class="keyword">double</span></code> precision
        internally. Will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">promote_float</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span></code>
        which in turn inherits from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">,</span>
        <span class="identifier">B</span><span class="special">&gt;</span></code>.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">promote_double_type</span>
</pre>
<p>
        Specifies whether or not to promote <code class="computeroutput"><span class="keyword">double</span></code>
        arguments to <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>
        precision internally. Will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">promote_float</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span></code>
        which in turn inherits from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">,</span>
        <span class="identifier">B</span><span class="special">&gt;</span></code>.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">discrete_quantile_type</span>
</pre>
<p>
        Specifies how discrete quantiles are evaluated, will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">discrete_quantile</span><span class="special">&lt;&gt;</span></code>
        instantiated with one of the <code class="computeroutput"><span class="identifier">discrete_quantile_policy_type</span></code>
        enumerated type.
      </p>
<pre class="programlisting"><span class="identifier">policy</span><span class="special">&lt;...&gt;::</span><span class="identifier">assert_undefined_type</span>
</pre>
<p>
        Specifies whether mathematically-undefined properties are asserted as compile-time
        errors, or treated as runtime errors instead. Will be an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">assert_undefined</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span></code> which
        in turn inherits from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">,</span>
        <span class="identifier">B</span><span class="special">&gt;</span></code>.
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;...</span><span class="identifier">argument</span> <span class="identifier">list</span><span class="special">...&gt;</span>
<span class="keyword">typename</span> <span class="identifier">normalise</span><span class="special">&lt;</span><span class="identifier">policy</span><span class="special">&lt;&gt;,</span> <span class="identifier">A1</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">make_policy</span><span class="special">(...</span><span class="identifier">argument</span> <span class="identifier">list</span><span class="special">..);</span>
</pre>
<p>
        <code class="computeroutput"><span class="identifier">make_policy</span></code> is a helper function
        that converts a list of policies into a normalised <code class="computeroutput"><span class="identifier">policy</span></code>
        class.
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A1</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A2</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A3</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A4</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A5</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A6</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A7</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A8</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A9</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A10</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A11</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A12</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">,</span>
          <span class="keyword">class</span> <span class="identifier">A13</span> <span class="special">=</span> <span class="identifier">default_policy</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">normalise</span>
<span class="special">{</span>
   <span class="keyword">typedef</span> computed-from-template-arguments <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
        The <code class="computeroutput"><span class="identifier">normalise</span></code> class template
        converts one instantiation of the <code class="computeroutput"><span class="identifier">policy</span></code>
        class into a normalised form. This is used internally to reduce code bloat:
        so that instantiating a special function on <code class="computeroutput"><span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">&gt;</span></code> or
        <code class="computeroutput"><span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">A</span><span class="special">&gt;</span></code> actually both generate the same code
        internally.
      </p>
<p>
        Further more, <code class="computeroutput"><span class="identifier">normalise</span></code> can
        be used to combine a policy with one or more policies: for example many of
        the special functions will use this to set policies which they don't make
        use of to their default values, before forwarding to the actual implementation.
        In this way code bloat is reduced, since the actual implementation depends
        only on the policy types that they actually use.
      </p>
</div>
<div class="copyright-footer">Copyright © 2006-2021 Nikhar Agrawal, Anton Bikineev, Matthew Borland,
      Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert Holin, Bruno
      Lalande, John Maddock, Evan Miller, Jeremy Murphy, Matthew Pulver, Johan Råde,
      Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg, Daryle
      Walker and Xiaogang Zhang<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div>
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